1. Field of the Invention
This invention relates to pacemakers or other implantable medical devices which utilize voltatile memory for the storage of information, and more particularly, to circuitry for both controlling access to the memory and for altering the state of memory under conditions of reduced power.
2. Description of the Prior Art
Cardiac pacemakers for supplying electrical stimulation of the heart in the absence of natural heartbeats are well-known in the art. Traditionally, these pacemakers have been manufactured utilizing discrete analog circuitry. However, more recently designed pacemakers utilize monolithic digital circuitry of great complexity. This additional capability has been exploited to implement desirable features such as programmability which enables the attending physician to non-invasively alter the operating parameters of the pacemaker. Typically, these programmable pacemakers use erasable or volatile memory to store the programmed information. See, for example, U.S. patent application Ser. No. 91,279 filed Nov. 5, 1979 to McDonald et al which discloses a pacemaker having a 23-bit memory for storing a digital word, used to control pulse width and pacing rate, among other variables. This digital word enters the pacemaker through a telemetry system which WRITES the digital word into volatile memory. In operation, logic within the pacemaker periodically READS the value of the word and uses this information to govern the operation of the pacemaker. The contents of the memory are accessed through an address decoder network which identifies the memory cells which will be read from or written to. During the WRITE operation the selected memory cell is addressed through the address decoder network and the appropriate logic level of the data to be stored is supplied to the WRITE input of the memory cell. After the data is presented to the memory cell a control signal is developed and applied to an ACCEPT input which permits the data to be transferred from the WRITE input into the memory flip-flop. In this context the term flip-flop includes any structure which provides a bistable structure for storing data. Consequently, it is important to preserve the integrity of this information during all operating conditions of the device.
The problem of maintaining the integrity of data stored in volatile memory is made difficult both by the nature of the power supply powering the pacemaker and by the existence of electromagnetic interference from cautery or defibillation within the pacemaker's operating environment.
Programmable pacemakers are typically powered by a primary battery which depletes during the operating life of the pacemaker. This depletion process is accompanied by a general increase in the internal battery impedance and a decline in the output voltage of the battery. These power supply characterisitics may result in a reduction in the power available to the memory circuitry of the pace-maker during certain operating conditions. If a WRITE operation is initiated when the power level is below a predetermined minimum safe operating level, an erroneous data transfer may occur and the previously stored information will be lost. Likewise, if the reduced power condition extends for a long period of time, the information contents of the memory cells may be lost due to leakage currents in the memory cells and the integrity of the data may be compromised.
Consequently, it is desirable to protect the integrity of information stored in memory by preventing WRITE operations from occuring during periods of low power. Also, it is desirable to force the contents of memory to some known configuration if the low power condition exceeds some predetermined time interval.